|Male of Megasoma actaeon|
|Female of Megasoma actaeon|
The species name actaeon derives from the name Actaeon of a famous Theban hero, son of the priestly herdsman Aristaeus and Autonoe in Boeotia, trained by the centaur Chiron.
Actaeon beetle is one of the largest of all beetles, measuring up to 7 centimetres (2.8 in) across, with a body length of about 5–12 centimetres (2.0–4.7 in). The males can grow to be 13½ cm (5.4 in) long by 4 cm (1.6 in) thick .
The dorsal surfaces are glabrous, matte or shiny black. The legs are powerful, with large tarsal claws. Males have appendages resembling horns on the pronotum and on the head. The two parallel horns on the pronotum are short, acute and forward pointing, while a much longer horn with a small tooth is present on the head. In the females the pronotum and the elytra are rugose and the horns are missing.
Females lay eggs into soil and eclosion and eggs develop in about 30 days. Larval development and pupation takes in total almost 3 years. Larvae at the 3rd instar can reach the weight of 200 grams, the highest weight among insects. Life expectancy of adults in captivity reaches about 100–150 days. The heaviest actaeon ever recorded was found in the northern regions of South America weighed 228 grams (8.04 oz) in 2009 nearly equivalent to a female rat. This not only makes it the heaviest beetle but the heaviest insect of any kind.